The invention generally relates to a method and a system for decreasing the concentration of molybdenum, tungsten, or both in aqueous solutions, and more particularly for precipitating molybdenum, tungsten, or both from aqueous solutions.
A need exists for decreasing the concentration of dissolved molybdenum and/or dissolved tungsten in aqueous solutions for several different purposes. Examples of some purposes include the purification of wastewater, groundwater, drinking water and process plant water, the refining of metals and the facilitation of chemical analytical determinations.
Although a variety of existing processes are known for decreasing the concentration of molybdenum in aqueous solutions, the existing processes suffer from one or more disadvantages. Existing processes for decreasing the concentration of molybdenum in aqueous solutions often require expensive chemical reagents, excessive quantities of chemical reagents and/or toxic chemical reagents. Examples of known processes include co-precipitation with ferric hydroxide, ferrous hydroxide ammonium molybdate precipitation, molybdenum sulfide precipitation and precipitation using organic compounds (see Dannenberg et al., “Molybdenum Removal from Concentrator Waste Water”, U.S. Bureau of Mines Report, July 1982, U.S. Pat. No. 4,211,753 to Pemsler and Litchfield; Beckstead et al., “Precipitation of Molybdenum Sulfide from Aqueous Solution”, JOURNAL OF METALS, July 1985 and Heininger and Meloan, “A Selective Reagent for the Removal and Recovery of Chromate, Molybdate, Tungstate, and Vanadate from Aqueous Solution”, SEPARATION SCIENCE AND TECHNOLOGY, 27, 1992, pp. 663-669).
Other examples of known processes to decrease the concentration of molybdenum in aqueous solutions do not involve precipitation, such as ion exchange and adsorption (see, e.g., U.S. Pat. No. 3,553,126 to Oberhofer; Wing et al. “Preparation of Insoluble Cationic Starches and their Use in Heavy Metal Anion Removal”, JOURNAL OF APPLIED POLYMER SCIENCE, 22, 1978, pp. 1405-1416; Zhao et al., “Removal of Molybdate and Arsenate from Aqueous Solutions by Flotation”, SEPARATION SCIENCE AND TECHNOLOGY, 31, 1996, pp. 769-785).
Few processes are known for decreasing the concentration of tungsten in aqueous solutions.
There exists a need for a method and a system for decreasing the concentration of molybdenum and/or tungsten in aqueous solutions that have at least one of the following advantages: no requirement for expensive chemical reagents, no requirement for excessive quantities of chemical reagents and no requirement for toxic chemical reagents.